This invention relates to a method and apparatus for the manufacture of wood chip boards in general and more particularly to a method and apparatus that allows for the manufacture of wood chip boards of varying width.
An apparatus for the manufacture of wood chip boards is disclosed in DE-PS 23 55 797. An apparatus of this type represents a considerable investment and it is therefore desirable not only to be able to produce boards with widths corresponding to the nominal working width of such an apparatus, but also, if necessary, boards of a smaller width. For example, board widths of 210 and 185 cm are conventional on the market. In an attempt to manufacture the smaller board width on an apparatus designed for the larger board width, the width of the filling must be made correspondingly smaller.
When this was done on apparatuses of the prior art problems occurred because the edges of the forming belts projecting out beyond the edge of the filling were not subjected to a counter-pressure and could not be adequately pressed against the support structure from which not only the pressure, but also the heat, is transmitted to the forming belts. Consequently, for the construction disclosed in DE-PS 23 55 797, the edge of the forming belts had no contact with the support structure or the rollers, which cover the entire width of the forming belts and transmit the heat from the support structure to the forming belts. As a result, the temperature of the forming belts dropped considerably near their edges. Since the wide middle zone of the forming belts was at a higher working temperature than the edge, there was considerable thermal stress, forcing the edge zone to contract longitudinally.
Such thermal stress became critical in the area of the reversing drums, because in addition to the thermal stress there were additional stresses due to the considerable longitudinal tension of the forming belts, as well as the stress resulting from the elongation of the outer fiber due to the belt reversal. The resulting total tensile stress on the outside of the zones of the forming belts wound over the reversing drums approached and in some cases exceeded, the yield stress. In either case this total tensile stress led to problems during continuous operation, particularly since the forming belts are made of stainless steel, which is not particularly resistant to repeated stress due to flexing.
Similar problems occurred in double-belt presses of the prior art even when the nominal width was used. These problems occurred because the filling did not extend to the edge of the forming belts, which projected transversely beyond the filling and also beyond the edge of the zone covered by the rollers. Here again there were temperature drops with the corresponding stresses.
In the press disclosed in DE-PS 22 43 465, an attempt was made to keep the temperature drop within certain limits by heating the projecting edges of the forming belts. However, it was found to be necessary to heat the edges of the forming belts over practically their entire length, because otherwise the temperature immediately dropped off beyond the heated zone. Heating the entire length of the forming belts, however, gave rise to considerable structural problems. For this reason, as well as because of the considerable expense, this press is generally not used.
As disclosed in DE-PS 28 19 943, another solution to the problem involved corrugating the projecting edge of the forming belts so that more material was available at the edge. Thus, in the event of a temperature drop near the edge, the longitudinal tensile stress resulting from the thermal contraction was not so great. Although this solution is practical for projecting edges of just a few centimeters in width, it is impractical if there is a temperature drop over projecting edges that are several tens of centimeters in width.
In view of the prior art, there is a need to develop a method and an apparatus of the type mentioned in the beginning so that board-producing webs of smaller width can be run on a double-belt press which has a predetermined nominal working width.